Electrical inductive apparatus having liquid and solid dielectric means

ABSTRACT

Electrical inductive apparatus of the type which includes an insulating structure comprising liquid dielectric means in series with solid insulating means. The solid insulating means is formed of a first organic resin and a filler which includes a second organic resin. The first organic resin is selected to provide the required chemical resistance and mechanical properties and the second organic resin is selected to have a dielectric constant which is lower than the first organic resin, to provide solid insulating means having a composite dielectric constant which more closely matches that of the liquid dielectric means than that of the first organic resin alone.

United States Patent References Cited [72] lnventor ThomasW.DakinMurrysville, Pa.

S m T m N u E m T m A P. "h" m w n n ma Am TS n c h t oa EJCK mwwm 999NH 269 684 64 000 353 333 n .m m w r o C .m r t m E 7 www 2 1 3 in 94hmw Q e N mm Nwmm p ms AFPA nv sv s 2247 Pittsburgh, Pa. PrimaryExaminer-Thomas J. Kozma Attorneys-A. T. Stratton, F. E. Browder and D.R. Lackey ABSTRACT: Electrical inductive apparatus of the type whichincludes an insulating structure comprising liquid dielectric means inseries with solid insulating means. The solid insulats4 ELECTRICALINDUCTIVE APPARATUS HAVING :Efl'TT'? j first 'f f-'" and 3 LIQUIDANDSOUD DIELECTRIC MEANS c u cs a sccon organic res m. e lrst organicresin is selected to provide the required chemical resistance and l 1Claims, 4 Drawlng Flgs.

mechanical properties and the second organic resin is selected 336/58,to have a dielectric constant which is lower than the first organicresin, to provide solid insulating means having a com- 252/63, 252/64, 3nus {555 94,

posite dielectric constant which more closely matches that of the liquiddielectric means than that of the first organic resin alone.

[50] Field of PATENIEU um 5 |97l SHEET 1 OF 2 FIG.I

FIG. 3

MU m Cmwmo v: .mm ZO 420x00 DIELECTRIC CONSTANT OF BARRIER FIG. 2

ELECTRICAL INDUCTIVE APPARATUS HAVING LIQUID AND SOLID DIELECTRIC MEANSBACKGROUND OF THE INVENTION 1 Field of the Invention The inventionrelates in general to electrical inductive apparatus, such astransformers and reactors, and more specifically to electrical inductiveapparatus of the liquid insulated and cooled type.

2. Description of the Prior Art Electrical inductive apparatus, such astransformers and reactors, conventionally utilize insulating structureswhich include combinations of oil filled ducts and pressboard barriersin series. In such a structure, the dielectric stress is distributedacross the oil'and solid insulation inversely proportional to thecapacitance of the two materials. Since the capacitance of the materialsis directly proportional to their dielectric constants, the voltagedistribution, and thus the electrical stress across the materials, isgreatly affected by the dielectric constants of the two materials.

The liquid dielectric, conventionally mineral (petroleum) oil, is usedto provide electrical insulation as well as to cool the inductiveapparatus. In order to cool the apparatus, the oil must be free tocirculate within the tank of the apparatus and through external heatexchangers, if used, and it must circulate as closely as possible to thesource of the heat. Since the electrical strength of oil drops as thewidth dimension of the oil duct increases, oil spaces are conventionallybroken into a plurality of small gaps or ducts, in order to optimize theelectrical strength of the oil.

Transformer oil has a dielectric constant which is usually in the rangeof 2.0 to 2.2, while oil impregnated pressboard has a dielectricconstant of about 3.75, or higher. Thus, electrical stress distributedacross pressboard barriers and oil gaps in series, concentrates in theoil filed gaps or ducts. This unequal sharing of the stress is furthercomplicated by the fact that the electrical breakdown strength of oil islower than that of the oil impregnated pressboard. Thus, in conventionalinsulating structures, the weaker material electrically is stressedhigher than the stronger" material. This results in ineffectiveutilization of the electrical characteristics of the solid insulation,requiring greater spacing between the windings of the apparatus, andbetween the windings and ground, than would be required if a morefavorable ratio of the dielectric constants of the solid and liquidmaterials could be achieved. Thus, a more effective utilization of theelectrical characteristics of the solid insulation would enableinsulating clearances to be reduced, which would reduce the length ofthe mean winding turn and the length of the magnetic circuit, resultingin a reduction in the size, weight and manufacturing cost of theelectrical apparatus.

SUMMARY OF THE INVENTION Briefly, the present invention is new andimproved electrical inductive apparatus of the liquid insulated andcooled type, which incorporates an insulating structure including solidinsulation and oil in series. However, instead of using pressboard forall the solid insulating members of the structure, at least certain ofthe solid insulating members are cast or molded to the desired shapefrom synthetic organic resins. The solid insulating members must bestable in oil at the elevated operating temperature of the oil, and theymust retain their mechanical strength, as these insulating members areoften used to support part or all of the weight of the windings; and, inorder to eliminate the disadvantages of oil impregnated pressboard, thesolid insulating members must have a lower dielectric constant than oilimpregnated pressboard. Organic resins which have dielectric constantswhich closely match the dielectric constant of oil. are, in generalunsuitable as they have marginal retention of mechanical strength at theelevated operating temperature of the oil. The oil attacks the resins,causing them to swell and weaken mechanically.

On the other hand, high strength, oil resistant casting or moldingresins which are suitable mechanically have dielectric constants in therange of 3.0 to 5.0, depending upon the concentration of polar groups inthe resin structure. Thus, they would have the same disadvantages as oilimpregnated pressboard, as their dielectric constants are in a similarrange.

The present invention utilizes solid insulating members cast or moldedof a resin system which includes a main or base or ganic resin, and afiller which is also an organic resin. The base resin is athermosettable resin, such as an epoxy or thermosettable polyester,preferably having a dielectric constant on the low side of thehereinbefore mentioned 3.0 to 5.0 range. The organic filler resin may bethermosettable or thermoplastic, and is selected to reduce the overalldielectric constant of the resulting therrnoset solid, below that of thefirst or base organic resin. The organic filler is used only as afiller, and thus may be one of the less costly resins having arelatively low dielectric constant, such as one of the hydrocarbons. Thepolyolefins are specially suitable, even though they are thermoplasticmaterials having little mechanical strength at the elevated operatingtemperature of the transformer oil, as they are inexpensive and have adielectric constant in a range of 2.2 to 2.6. The concentration of theorganic filler may be in the range of 20 to 60 percent by volume of theresulting thermoset solid, and is preferably about 50 percent of thevolume. Since the sole purpose of the organic filler is to lower thecomposite dielectric constant of the resulting resinous system, themaximum amount of filler is a practical one, being that amount whichwill not reduce the mechanical strength and oil resistance of theresulting resinous system below that necessary for the solid insulationsystem to perform its required functions in the oil filled electricalapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of theinvention will become more apparent when considered in view of thefollowing detailed description of exemplary embodiments thereof, takenwith the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of an insulating structure subjected to anelectrical stress which simulates many of the insulating structures usedin liquid cooled electrical inductive apparatus;

FIG. 2 is a graph which illustrates how the electrical strength of aninsulating structure comprising a solid barrier in mineral oil varieswith the dielectric constant of the barrier;

FIG. 3 is diagrammatic view of the test arrangement used to obtain thecurves shown in the graph of FIG. 2; and

FIG. 4 is a perspective view, partially cut away, of a liquid filledtransformer which may utilize the teachings of the inventlon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,and FIG. 1 in particular, there is shown a diagrammatic representationof a typical insulating structure in oil cooled electrical inductiveapparatus, such as transformers and reactors of both the shell andcoreform type. The conventional pressboard-oil type of insulatingstructure is represented by block 10, which includes blocks 12 and 14disposed in series between electrodes 16 and 18. Electrodes 16 and 18are connected to a source 20 of alternating potential, thus creating anelectrical stress across the insulating structure 10. Block 10represents transformer (mineral) oil having a dielectric constant ofabout 2. and block 14 represents oil impregnated pressboard, acellulosic insulation having a dielectric constant of about 4. For themost effective utilization of the insulating structure l0, which wouldallow the dimension D to be reduced to a minimum, each of the elementsin series between the electrodes should be stressed to a predeterminedpoint dependent upon its electrical strength. Since oil impregnatedpressboard has a higher dielectric strength than mineral oil, forefficient use of the insulating structure it should be stressed higherthan the oil l2. However,

since the voltage distribution across two dissimilar dielectricmaterials in series is inversely proportional to the capacitance of thetwo material, the voltage distribution is affected by the dielectricconstants of the materials. As shown in FIG. 1, if the boundary 22between the oil and pressboard insulations l2 and 14 is midway betweenthe electrodes 16 and 18, twothirds of the voltage will appear acrossthe oil 12 and only onethird across the solid insulation 14, due totheir different dielectric constants. Thus, instead of transferring thestress to the solid electrical insulation 14, which has the higherelectrical strength, stress is transferred to the oil 12, which has thelower electrical strength of the two materials, and which is subject tocontamination which will lower its electrical strength still further.Moving the boundary line 22 closer to electrode 16, to achieve the samevoltage drop across each of the materials is not a practical solution,as regardless of where the boundary 22 is located, the voltage gradientat the boundary will be twice as high in the oil 12 as in the solidinsulation 14, due to the relationship of their dielectric constants.

Since the use of mineral oil and pressboard in insulation structures hasbeen the most economical and practical approach to insulating powertransformers the disadvantages of their combination, hereinbefore setforth, have been overcome by adjusting or designing the insulatingclearances in the apparatus to compensate for their different dielectricconstants. It would be desirable to more effectively utilize theinsulating structures in liquid filled inductive apparatus, which wouldenable the insulating clearances between the high and low voltagewindings, between the coils of a winding, between electrical phases, andbetween the windings and ground, to be reduced, thus reducing the meanlength of the coil turns, and the length of the magnetic circuit, whichreduces the required amount of copper or aluminum, and magnetic coresteel. Reducing the physical size of the magnetic core-winding assemblyreduces the size of enclosing tank, and the amount of oil required tofill the tank. Thus, the size, weight and cost of the inductiveapparatus may be reduced by lowering the dielectric constant of thesolid insulation.

FIG. 2 is a graph containing curves 22 and 24 which illustrate how theelectrical breakdown strength of insulating structures containing solidinsulation and mineral oil may be substantially increased by loweringthe dielectric constant of the solid insulation. The dielectric constantof a solid insulating barrier or member is listed on the abscissa whilethe on-set of conrona in kilovolts (kv.), measured from the crest of thevoltage wave, is listed on the ordinate. Curve 22 is for an insulatingbarrier having a thickness dimension of 0.0625 inch, while curve 24 isfor an insulating barrier having a thickness dimension of 0.125 inch.The test data for curves 22 and 24 was obtained from a test arrangementas shown in FIG. 3, which diagrammatically illustrates a barrier 25having a thickness dimension 1 disposed in mineral oil, with electrodes26 and 27 disposed to contact opposite sides of barrier 25. Electricalbreakdown occurs in the areas 28, which, due to the fringing effect ofthe electrical field, places the oil in these areas in series with thebarrier 25. It will be noted from curve 22 in F IG. 2, that with abarrier having a thickness dimension of 0.0625 inch, that the coronaon-set level is increased from about 38 kv. to 55 kv. by reducing thedielectric constant of the barrier from 4 to 2. Curve 24 illustratesthat increasing the thickness of the barrier to 0.125 inch raises theelectrical strength of the oil barrier combination. With the thickerbarrier, the corona on-set level is increased from about 55 kv. to about76 kv. by reducing the dielectric constant of the barrier from 4 to 2.

Ideally, the solid insulation and the oil should be stressed accordingto their electrical strengths. This would require the solid insulationto have a lower dielectric constant than the oil. In substituting asolid insulating material for the pressboard, however, the dielectricconstant of the new material is only one of the factors which must beconsidered. For example, the cost of the solid insulating material mustnot be so much greater than the pressboard that the savings achieved byreducing insulating clearances are offset by the additional cost of thesolid insulation. Cost is also one of many reasons why the problem hasnot been attacked by using a liquid dielectric having a higherdielectric constant. The synthetic liquid dielectrics have a higherdielectric constant than mineral oil, but their cost is alsosubstantially higher. The high cost of the synthetic liquids comparedwith the cost of mineral oil, has limited the use of these syntheticmaterials to applications where the fire hazard of mineral oil is adetermining'factor in the application.

The solid insulating material substituted for pressboard, or used incombination therewith, must be compatible with mineral oil, it must havesufficient mechanical strength to provide support for the windings, andthis strength must not be adversely affected at the maximum operatingtemperature of the liquid dielectric. Finally, the solid insulatingmaterial substituted for pressboard, or used in combination therewith,must have a high electrical strength.

The present invention provides a solid insulating system having adielectric constant sufficiently lowerv than that of oil impregnatedpressboard, that a reduction in total spacing as much as 30 percent maybe obtained, which substantially reduces the manufacturing cost of theapparatus.

More specifically, the present invention is electrical inductiveapparatus of the liquid cooled type, which utilizes insulatingstructures which include solid insulation and oil filled ducts in seriesbetween electrically stressed parts of the apparatus, with at leastcertain of the solid insulating members being molded or cast of asynthetic resin system. The synthetic resin system includes first andsecond organic resins. The first organic resin is selected for itsmechanical strength, electrical strength, and resistance to attack fromhot transfonner oil. Since synthetic resins of this nature, in general,have a dielectric constant which is about the same, or higher than thatof oil impregnated pressboard, their use as a direct substitute forpressboard is not economical, as the insulating clearances would remainthe same. The second organic resin is selected primarily for itsdielectric constant, i.e., for its relatively low dielectric constant,and is used as a filler for the first resin to provide a compositedielectric constant for the resulting resinous system which issubstantiallymtidway between that of the first and second resins. Thesecond organic material is used primarily as a filler, and not tocross-link the first resin, with the maximum amount of the second resinused being that amount which will still enable the first resin to meetits requirements of mechanical and electrical strength as well asresistance to hot transformer oil. Thus, the concentration of the fillerisnt critical, with at least 20 percent by volume being about theminimum, and 50-60 percent being a practical maximum. The practicalmaximum is determined by the ability to obtain a uniform dispersion ofthe filler throughout the resin system, as well as the ability to moldor cast structures having the requisite mechanical characteristics. ltis very important that a uniform dispersion of the tiller be obtained,and that the filler particles are set by and completely surrounded bythe thermosettable base resin. Otherwise. the oil may come into contactwith the filler particles, causing them to swell.

The first organic resin system is preferably thermosettable, since itmust retain its mechanical strength and stability in transformer oil atelevated temperatures, such as 1 l0 to l30 C., or higher. Suitableresins are the unsaturated linear polyester resins, which are capable ofcross-linking with a monomer to form a thennoset solid polymer, andresins of the epoxy type. Resins of these types are commerciallyavailable which oil resistant, have excellent mechanical and electricalstrengths at ambient and at the elevated operating tempera tures of theelectrical apparatus, at a cost which will not offset potential savings.Both of these types of resins may be cast or molded to shape. Thedielectric constants of suitable epoxy and polyester resins are in therange of 3 to 5, with resins having dielectric constants on the lowerside of this range being preferred.

The second organic resin should be relatively inexpensive, such as oneof the hydrocarbons, it should have a melting point greater than about150 C., and it should have as low a dielectric constant as possible.Since the first organic resin satisfies the oil resistance andmechanical strength requirements of the insulation, the second resinneed not have appreciable mechanical strength at the elevated operatingtem perature of the electrical-apparatus. The polyolefins, such aspolypropylene and high density (isotactic) polyethylene, have been foundto be excellent, due to their low dielectric constants [about 2.2],their relatively low cost, and their high melting points. By usingpolypropylene or high density polyethylene in an epoxy or polyesterresin system, with the epoxy or polyester resin system selected to havea dielectric constant which is on the low side of the available range, acomposite dielectric constant in the range of 2.5 to 3 is achievable.For example, epoxy resin with 43 percent by weight polypropylene powderfiller provides a dielectric constant of 2.7. lsotactic polystyrene andpolymethyl pentene may also be used as the filler resin.

The filler resin is preferably in the form of a fine powder. Theparticle size of the powder, however, is not critical, with uniformdispersion being the important factor. In general, powders of 100 mesh,or finer, are preferred, as it is easier to uniformly mix them withliquid epoxy or polyester resin, but coarser powders may be used ifdesired.

The filler particles must be separated, and each encapsulated in thebase resin, to prevent ingress of oil and resulting swelling of thefiller. Wetting agents, such as those of the type which have a polargroup on one end of the molecule, and a hydrocarbon chain on the other,may be used to insure wetting of the filler 64 by the base resin.

FIG. 4 is a perspective view, partially cut away, of a transformer whichmay utilize the teachings of the invention. Transformer 30 is of theshell-form type, having a magnetic core-winding assembly 32 disposed ina tank 34. Tank 34 is filled to a level 36 with mineral oil, such asl-lumbles Univolt 33, with the magnetic core-winding assembly 32 beingcompletely immersed in the oil. The oil aids in insulating theelectrical windings from ground, and from one another, and it alsoserves to cool the transformer. l-leat exchangers or coolers [not shown]are connected to the tank, in communication with the openings 38 nearthe top of the tank 54, and the pipes 40 near the bottom thereof, withthe oil circulating through the tank and through the ducts in themagnetic corewinding assembly 32, either by thermal syphon or by forcedcirculation, to remove heat from the oil picked up from the magneticcore-winding assembly 32.

Transformer 30, which may be single or polyphase, has a plurality ofhigh and low voltage coils which encircle the leg portions of magneticcore sections 39 and 41. The coils are disposed in side-by-siderelation, separated by solid insulating barriers. A group 42 of lowvoltage coils is disposed between barriers 44 and 46, and a group 48 oflow voltage coils is disposed between barriers 48 and 50, while a group52 of high voltage coils is disposed between barriers 46 and 48. Thebarriers 44, 46, 48 and 50 have a plurality of spacer blocks 54 attachedthereto for providing cooling ducts for the oil.

The electrical coils within each of the groups are also separated byinsulating structures which include solid insulating members and spacerswhich provide oil ducts therein.

The solid barriers between groups of coils, such as barriers 44, 46, 48and 50, may all be constructed according to the teachings of theinvention, or combination of pressboard and synthetic cast or moldedmembers may be used. Additional savings may be realized by integrallycasting or molding the spacer blocks 54 with the barriers, if desired.The costly conventional practice of manually gluing insulating spacerblocks to sheets of pressboard may thus be eliminated. The solid spacermembers within the groups of coils may also be constructed by casting ormolding, using the low dielectric constant resin system disclosedherein, as well as insulating channels and other insulating memberswhich are in series with the oil across highly stressed locations in thetransformer.

While transformer 30 is of the shell-form type it is to be understoodthe invention may be applied to single and polyphase transformers of thecore-form type, as well as any electrical apparatus of the type whereinsolid insulation and oil are disposed in series between two electrodesof different potential. For example, in core-form transformers, solidinsulation, cast or molded of a low dielectric constant resin system,may be used as insulating barriers between phases, between the windingsand phases to ground, between the ends of the windings and phases toground, between the ends of the windings and the pressure rings, and inthe high-low space between the high and low voltage coils of anelectrical phase.

While the cast or molded low dielectric constant resin system disclosedherein would be used to replace pressboard or other cellulosicinsulation, or used in combination therewith, it does not necessarilyhave to be formed in the same shapes. Pressboard for example, isavailable only in relatively thin sheets, which are shaped and stackedto form the desired insulating structure at a considerable additionalassembly cost. Since the solid insulation of the present invention iscast or molded to shape, thicker sections and a greater variety ofshapes may be easily formed.

The invention has been described primarily for use in mineral oilinsulated electrical inductive apparatus, but it will be obvious that itmay be used with any insulative liquid when a more favorabledistribution of electrical stress may be obtained by reducing thedielectric constant of solid insulating members.

In summary, there has been disclosed electrical apparatus of the typewhich utilizes new and improved solid insulating members and oil inseries between two electrodes of different electrical potential. Thedisclosed teachings enable the size and cost of such apparatus, such astransformers and reactors, to be substantially reduced, as it permitssmaller spacings between the coils and windings, between electricalphases, and between the windings and ground, by significantly reducingelectrical stress in oil filled ducts. For example, an insulatingstructure having equal amounts of pressboard and oil, with thepressboard having a dielectric constant of 3.75, in series with thecoil, which has a dielectric constant of 2.2, in a uniform electricalfield, results in the stress in the oil being 1.7 times the averagestress. In a cast or molded resin system having a composite dielectricconstant of 2.7 is used in the same proportions with the oil, the stresson the oil will only be 1.2 times the average stress. This reduction instress permits a reduction in total spacing of up to 30 percent, withoutincreasing the stress in the oil. Some voltage stress has beentransferred to the solid insulation, which has a higher electricalstrength than the oil, resulting in a more efficient use of theinsulating materials, and a substantial reduction in the size, weightand cost of the apparatus.

I claim as my invention:

1. Electrical apparatus comprising:

an enclosure;

liquid dielectric means having a predetermined dielectric constant, saidliquid dielectric means being disposed in said enclosure;

an electrical winding disposed in said enclosure and immersed in saidliquid dielectric means;

an insulating structure comprising solid insulating means and saidliquid dielectric means in series, said insulating structure providingat least a portion of the insulating system for said electrical winding;

said solid insulating means including a first organic resin having adielectric constant greater than that of the liquid dielectric means,and filler means comprising a second organic resin having a dielectricconstant lower than that of the first organic resin, to provide acomposite dielectric constant for said solid insulating means which moreclearly matches the dielectric constant of said liquid dielectric meansthan that of the first organic resin alone.

2. The electrical apparatus of claim 1 wherein the filler means iscompletely encapsulated by the first organic resin.

3. The electrical apparatus of claim 1 wherein the first organic resinis an epoxide and the second organic resin is a polyolefin.

4. The electrical apparatus of claim 3 wherein the polyolefin isisotactic polyethylene.

5. The electrical apparatus of claim 3 wherein the polyolefin ispolypropylene.

6. The electrical apparatus of claim 1 wherein the first organic resinis a thermoset polyester and a second organic resin is a polyolefin.

7. The electrical apparatus of claim 6 wherein the polyolefin isisotactic polyethylene.

8. The electrical apparatus of claim 6 wherein the polyolefin ispolypropylene.

9. The electrical apparatus of claim 1 wherein the second organic resinoccupies 20 to 60 percent of the volume of the solid insulating means.

10. The electrical apparatus of claim 1 wherein the liquid dielectricmeans has a dielectric constant in the range of about 2 to 2.2, thefirst organic resin has a dielectric constant in the range of about 3 to5, a second organic resin has a dielectric constant in the range ofabout 2 to 2.3, and the composite dielectric constant of the solidinsulating means is in the range of about 2.5 to 3. p

11. The electrical apparatus of claim 11 wherein the liquid dielectricmeans is a mineral oil.

1. Electrical apparatus comprising: an enclosure; liquid dielectricmeans having a predetermined dielectric constant, said liquid dielectricmeans being disposed in said enclosure; an electrical winding disposedin said enclosure and immersed in said liquid dielectric means; aninsulating structure comprising solid insulating means and said liquiddielectric means in series, said insulating structure providing at leasta portion of the insulating system for said electrical winding; saidsolid insulating means including a first organic resin having adielectric constant greater than that of the liquid dielectric means,and filler means comprising a second organic resin having a dielectricconstant lower than that of the first organic resin, to provide acomposite dielectric constant for said solid insulating means which moreclearly matches the dielectric constant of said liquid dielectric meansthan that of the first organic resin alone.
 2. The electrical apparatusof claim 1 wherein the filler means is completely encapsulated by thefirst organic resin.
 3. The electrical apparatus of claim 1 wherein thefirst organic resin is an epoxide and the second organic resin is apolyolefin.
 4. The electrical apparatus of claim 3 wherein thepolyolefin is isotactic polyethylene.
 5. The electrical apparatus ofclaim 3 wherein the polyolefin is polypropylene.
 6. The electricalapparatus of claim 1 wherein the first organic resin is a thermosetpolyester and a second organic resin is a polyolefin.
 7. The electricalapparatus of claim 6 wherein the polyolefin is isotactic polyethylene.8. The electrical apparatus of claim 6 wherein the polyolefin ispolypropylene.
 9. The electrical apparatus of claim 1 wherein the secondorganic resin occupies 20 to 60 percent of the volume of the solidinsulating means.
 10. The electrical apparatus of claim 1 wherein theliquid dielectric means has a dielectric constant in the range of about2 to 2.2, the first organic resin has a dielectric constant in the rangeof about 3 to 5, a second organiC resin has a dielectric constant in therange of about 2 to 2.3, and the composite dielectric constant of thesolid insulating means is in the range of about 2.5 to
 3. 11. Theelectrical apparatus of claim 11 wherein the liquid dielectric means isa mineral oil.